Attitude polarization, also known as belief polarization, is a phenomenon in which a disagreement becomes more extreme as the different parties consider evidence on the issue. It is one of the effects of confirmation bias: the tendency of people to search for and interpret evidence selectively, to reinforce their current beliefs or attitudes.[1] When people encounter ambiguous evidence, this bias can potentially result in each of them interpreting it as in support of their existing attitudes, widening rather than narrowing the disagreement between them.[2]

The effect is observed with issues that activate emotions, such as political "hot button" issues.[3] For most issues, new evidence does not produce a polarization effect.[4] For those issues where polarization is found, mere thinking about the issue, without contemplating new evidence, produces the effect.[4] Social comparison processes have also been invoked as an explanation for the effect, which is increased by settings in which people repeat and validate each other's statements.[5]
This apparent tendency is of interest to psychologists, but also to sociologists[6] and philosophers.[7]

In 1979, Charles Lord, Lee Ross and Mark Lepper carried out an important piece of research on attitude polarization.[8] The researchers selected two groups of people; one group was strongly in favour of capital punishment, the other group were strongly opposed to capital punishment. The researchers began by measuring the strength with which people held their particular position on the death penalty. Later, both the pro- and anti-capital punishment people were put into small groups and shown one of two cards, each of which had a statement about the results of a research project written on it. For example:

Kroner and Phillips (1977) compared murder rates for the year before and the year after adoption of capital punishment in 14 states. In 11 of the 14 states, murder rates were lower after adoption of the death penalty. This research supports the
deterrent effect of the death penalty.[9]

or:

Palmer and Crandall (1977) compared murder rates in 10 pairs of neighboring states with different capital punishment laws. In 8 of the 10 pairs, murder rates were higher in the state with capital punishment. This research opposes the deterrent effect of the death penalty.[9]

The researchers again asked people about the strength of their beliefs about the deterrence effect of the death penalty, and, this time, also asked them about the effect that the research had had on their attitudes.

In the next stage of the research, the participants were given more information about the study described on the card they received, including details of the research, critiques of the research, and the researchers' responses to those critiques. The participants' degree of commitment to their original positions were re-measured, and the participants were asked about the quality of the research and the effect the research information had on their beliefs.

Finally, the trial was rerun on all the participants using a card that supported the opposite position to that they had initially seen.

The researchers found that people tended to hold that research that agreed with their original views had been better conducted and was more convincing than research that conflicted with their original views.[10] Whichever position they held initially, people tended to hold that position more strongly after reading about research that supported their position. Lord et al.. point out that it is reasonable for people to be less critical of research that supports their current position, but it seems less rational for people to significantly increase the strength of their attitudes when they read supporting evidence.[11] When people had read both the research that supported their current views and the research that was conflicted with their views, they tended to hold their original attitudes more strongly than before they received that information.[12]

Social psychologists have carried out research on the effect of seeing oneself as part of a group on one's attitude towards oneself, the group and positions supported or rejected by that group.[13] To briefly summarise, the research suggests that people are likely to accept the position that they believe their group holds, even when they have only just been put into the group and have yet to meet any of the other group members.[14]